Hi,
1) Yes you can put an entry in boxes 44 and 45 but not boxes 24-29, box 30, and boxes 32 to 36. The guidance is here:
UK property notes (2022)
2) No if you claim the property allowance you are not also allowed to claim individual expenses.
3) Mileage expenses would go in box 29 so are not allowed when you claim the property allowance. They can be calculated as by using the actual cost method or the fixed rate mileage
allowance. The guidance is here:
Property Income Manual
Thank you.